The overall goal of this project is to develop new and improved methodology with which to perform high throughput screening of G protein coupled receptors for drug discovery purposes. G protein coupled receptors include a large superfamily of cell surface proteins that are targets for many of the known drugs. Included in this class of receptors are orphan receptors whose natural ligands are unknown. Currently these receptors are difficult to screen. The proposed methodology will make it considerably easier to screen both known and orphan G protein coupled receptors. We propose coupling a pH-sensitive derivative of Green Fluorescent Protein to the amino terminus of several G protein coupled receptors. Upon activation, these receptors undergo internalization to endocytic membranes that presumably have a lower pH. If so, we expect to see changes in the fluorescence characteristics. If this works, we propose expanding this technology to many known and orphan receptors and eventually commercializing these assays to the drug discovery market. PROPOSED COMMERCIAL APPLICATION: NOT AVAILABLE